Situations exist in which it is desirable to isolate a metal structure from ground for low level DC voltages while allowing normal steady-state AC currents to pass freely. An example is cathodic protection for a metal structure to which a low level DC voltage is applied to protect the structure from corrosion. Such a protection system is commonly employed, for example, by power utilities to protect gas transmission and distribution lines, storage tanks, and metal coverings of high voltage cables. In such situations it is also desirable to effectively ground the structure to protect it from system faults which can otherwise cause large AC voltages to be present on the structure. The use of a conventional grounding system, in which the structure would be connected directly to ground, would, of course, result in the loss of DC isolation for the cathodically protected structure. Comparable problems are encountered where boats are supplied with shore based power. If a conventional third wire conductor is used to ground a metal hull or other underwater structure, a galvanic circuit is formed between the boat and shore located metal structures. If the boat hull is an aluminum hull, or the boat is equipped with aluminum outdrives or outboard motors, the aluminum tends to become a sacrificial anode which will corrode away.
For such situations, it is desirable to utilize a grounding system which prevents low levels of DC current from leaking into or out of the protected structure while at the same time allowing normal AC currents to freely pass from the structure to ground. It is also desirable that, in the event of a fault, the fault current will also be passed freely to ground, though DC isolation may be momentarily interrupted. Polarization cells have been developed to perform this function, but such cells use potentially hazardous electrolytes which can be harmful to equipment, personnel and the environment. Such polarization cells also require regular maintenance.
A solid state device performing a function similar to a polarization cell for marine ground isolation is shown in U.S. Pat. No. 3,636,409. This device provides a ground filter for a boat or other vessel which is obtaining electrical power from a shore-based source while at dock where it is desirable that the hull of the vessel be AC grounded to protect the crew from hazardous shocks resulting from faults in the AC power supply system. The device shown in U.S. Pat. No. 3,636,409 uses a capacitor to provide AC grounding. Two sets of diodes are connected in parallel with the capacitor to conduct in opposite directions, with each set being formed of two separate series connected diodes which block low DC voltages but pass large DC and AC fault currents. Other devices having a similar purpose are shown in U.S. Pat. Nos. 3,769,926, 3,930,977 and 4,117,345.
Electrical isolation devices are also used for purposes other than ground isolation. For example, such a device may be used to provide AC continuity between cathodically protected structures separated by an insulated joint, such as an insulated flange on a pipeline. The device would provide AC continuity to the structures while isolating them from each other with respect to DC.